Project Summary: Activity-dependent changes in gene expression underlie cognitive processes such as learning and memory. Mutations in components of the signaling network that controls the activity-dependent gene program can lead to a variety of neurological disorders including Rett Syndrome, Coffin Lowry Syndrome, and Rubinstein-Tabi Syndrome. Building on two decades of research into the mechanisms by which neuronal activity regulates gene transcription during nervous system development we have recently found that MEF2 family transcription factors act to restrict the number of excitatory synapses that form onto neurons. In this application we describe a series of experiments that will characterize the mechanisms by which MEF2 regulates synapse development and function. We propose the following specific aims: 1) To characterize new components of the signaling network that regulates MEF2 activity during the development and refinement of synaptic connections. We will seek to characterize the nature of neuronal MEF2 transcriptional complexes to gain a thorough understanding of the mechanisms by which neuronal activity regulates MEF2 function. 2) To investigate the importance of MEF2 as a regulator of synapse number in vivo by disrupting MEF2 function in organotypic cultures and in mice. 3) To employ genome-wide strategies to identify and characterize the MEF2 targets that control synapse number. It is our hope that the proposed experiments will provide a better understanding of the role of MEF2 in synapse development and may ultimately provide new insights into the importance of this activity-regulated gene program for both human cognition and disease. Relevance: New gene expression induced by neuronal activity is thought to mediate long-term changes in neuronal function, and mutations in components of this gene expression program cause profound defects in human cognitive function. This proposal seeks to investigate how a family of regulatory factors that controls gene expression leads to alterations in neuronal connectivity, as well as the potential relevance of this process to human cognition and disease.